Printers of known kind are provided with print mechanisms including print regions in which the printing operation is performed. Each such printer is supplied with a print medium on which the printing takes place. The print medium is commonly paper and for convenience the paper is formed into a continuous web. The web is fed through the print region of the printer by a suitable feeding mechanism. As the web is fed through the printer the print mechanism performs printing operations on the web. In order to ensure that the printing operations result in characters being printed in the correct positions on the paper it is essential that the paper be aligned correctly with the print mechanism throughout the printing operation.
The paper web is commonly fed through the printer by means of tractor mechanisms. These mechanisms include wheels formed with pins on their peripheries or pairs of wheels driving a belt formed with pins. The pins engage in holes formed in the web near the side edges of the web. Each tractor mechanism is driven by a suitable drive means and as the tractor wheels rotate or the belt moves the pins engage in successive holes in the web and feed the web through the printer.
The printer includes a platen in its print region where printing takes place. The paper web is fed over the platen. In known arrangements two tractors are located on the downstream side of the platen (in relation to the movement of the web) to draw the web through the print region over the platen during forward feed. It is necessary to ensure that the web is tensioned as it passes over the platen in order to ensure that printing takes place on a flat area of the web. In order to tension the web it is known to provide tensioners formed with spring biased arms which engage the surface of the web thereby retarding the movement of the web through the printer. These tensioners are located on the upstream side of the platen retarding the movement of the paper web as it is fed through the printer by the tractor mechanism. The tensioners thereby tension the web and ensure that the paper web is flat as it passes over the platen.
It is difficult to ensure that the tension exerted by the tensioners of this type is correct since it relies upon the friction between the spring biased arm and the surface of the web. This friction may vary from time to time with changes in the ambient conditions and will vary with different kinds of paper. Also with this type of tensioner it is not possible to provide for feed of the paper web in the reverse direction since the arm tends to damage the web.
In order to provide for reverse feed of the paper web it is known to replace the tensioners which have the spring biased arms by additional tractors located upstream of the platen. For forward feed of the paper web the tractors located downstream of the platen are driven and the tractors located upstream of the platen are not driven but are arranged to provide a drag which tensions the paper web. For reverse feed of the paper web the upstream tractors are driven and the downstream tractors are not driven. The downstream tractors may be arranged to provide a drag which tensions the paper web during reverse feed.
All the tractors include wheels or belts formed with pins which engage in holes in the web. These do not allow lateral movement of the web in the region of the tractors. If the web stretches or there is some misalignment between the upstream and downstream tractors the pins on the downstream tractors will try to position the web in one way and the pins on the upstream tractors will try to position the web in another way. As a result the web will be stressed and distorted and will not be correctly aligned in the print region.
While the background of the invention has been illustrated with reference to a printer it will be appreciated that the same problems will occur in the feeding of a web through other operating mechanisms, for example feeding a paper web through a punch mechanism or feeding a web through a mechanism for detecting marks on the web.
The object of the present invention is to provide apparatus for performing operations on a web having an operating mechanism and an improved web feeding mechanism for feeding a web past the operating mechanism which tensions the web as it passes the operating mechanism and allows for lateral alignment of the web with the operating mechanism under the control of only one part of the feeding mechanism.
A further object of the present invention is to provide apparatus of the above type having an improved web feeding mechanism for feeding the web in the reverse direction past the operating mechanism which maintains the web under tension.
A still further object of the present invention is to provide apparatus having an improved web feeding mechanism for tensioning the web during both forward and reverse feed of the web past the operating mechanism which does not damage the surface of the web.